18.6.5 AC_ACT_IFELSE vs. AC_TRY_ACT

Since Autoconf 2.50, internal codes uses AC_PREPROC_IFELSE,
AC_COMPILE_IFELSE, AC_LINK_IFELSE, and
AC_RUN_IFELSE on one hand and AC_LANG_SOURCE,
and AC_LANG_PROGRAM on the other hand instead of the deprecated
AC_TRY_CPP, AC_TRY_COMPILE, AC_TRY_LINK, and
AC_TRY_RUN. The motivations where:

a more consistent interface: AC_TRY_COMPILE etc. were double
quoting their arguments;

the combinatoric explosion is solved by decomposing on the one hand the
generation of sources, and on the other hand executing the program;

this scheme helps supporting more languages than plain C and C++.

In addition to the change of syntax, the philosophy has changed too:
while emphasis was put on speed at the expense of accuracy, today's
Autoconf promotes accuracy of the testing framework at, ahem..., the
expense of speed.

As a perfect example of what is not to be done, here is how to
find out whether a header file contains a particular declaration, such
as a typedef, a structure, a structure member, or a function. Use
AC_EGREP_HEADER instead of running grep directly on the
header file; on some systems the symbol might be defined in another
header file that the file you are checking includes.

As a (bad) example, here is how you should not check for C preprocessor
symbols, either defined by header files or predefined by the C
preprocessor: using AC_EGREP_CPP:

AC_EGREP_CPP(yes,
[#ifdef _AIX
yes
#endif
], is_aix=yes, is_aix=no)

The above example, properly written would (i) use
AC_LANG_PROGRAM, and (ii) run the compiler: